Run tests with escript

Using escript to run tests is faster (1 second instead of 30).
To integreate better with eunit, tests are now suffxed with _tests.erl.
word_count needed a change to compile.
bank_account and binary_string needed renaming to fit the pattern.
Add exit value to test script
Correct exit status
Merge pull request #2 from ebengt/check-exercises-escript

Check exercises escript